Board Thread:Location Help/@comment-31122670-20170826230713/@comment-31267872-20170827144340

They were probably talking about the Mental Herb, which can be ocassionally held by wild Lotad by surfing on Route 2, or they were talking about the Power Herb, ocassionally held by wild Nuzleaf on Route 16.

So, either the Mental Herb or Power Herb.